1. Field of the Invention
The present invention relates to an emergency head retract system for magnetic disc drives and, more particularly, to a system which takes advantage of the kinetic energy stored in a rotating spindle mass for providing the energy required to unload the heads in a disc drive in a power failure or other emergency situation.
2. Description of the Prior Art
Magnetic disc storage systems are widely used to provide large volumes of relatively low-cost, computer-accessible memory or storage. A typical disc storage device has a number of discs coated with a suitable magnetic material mounted for rotation on a common spindle and a set of transducer heads carried in pairs on elongated supports for insertion between adjacent discs, the heads of each pair facing in opposite directions to engage opposite faces of the adjacent discs. The support structure is coupled to a positioner motor, the positioner motor typically including a coil mounted within a magnetic field for linear movement and oriented relative to the discs to move the heads radially over the disc surfaces to thereby enable the heads to be positioned over any annular track on the surfaces. In normal operation, the positioner motor, in response to control signals from the computer, positions the transducer heads radially for recording data signals on, or retrieving data signals from, a pre-selected one of a set of concentric recording tracks on the discs.
The transducer heads are supported above the disc surfaces by a film of air to prevent contact therebetween which might otherwise damage one or both members. The heads are typically designed to actually fly above the disc recording surfaces at heights of less than 50 microinches. Irreparable damage can result from an electrical power failure which slows the discs and allows the heads to settle into contact with the discs' surfaces. As a result, it is imperative that the heads be withdrawn from the vicinity of the discs if the disc rotational speed is reduced substantially. It is also important, in a removable media disc drive, to insure that the heads are removed from the vicinity of the disc surfaces in the event of a power failure so that the disc can be removed from the system without damaging the heads.
The process of removing the heads from the discs in an emergency situation is referred to as an "emergency unload procedure" and requires the heads to be moved radially toward the discs' outer tracks and axially away from the disc surfaces. Although loss of power is probably the primary reason for initiating an emergency unload procedure, the procedure is typically also initiated when disc speed does not remain within tolerance, positioner error is detected, or write circuit faults that could affect stored data are detected.
Essentially, all modern disc drives incorporate some system for executing an emergency unload procedure in order to avoid loss of data and prevent disc and/or head damage. In a typical prior art emergency unload system, a capacitor is charged by the drive power supply during normal operation. Upon the detection of an emergency condition, a relay or equivalent switching means switches the capacitor across the positioner coil terminals to provide the electromotive force necessary to move the head support structure across the disc surfaces. Upon approaching the disc outer edge, the head support structure encounters a mechanical ramp which imparts an axial force to the support structure, thus unloading the heads from the discs. Systems of this type are described and claimed in U.S. Pat. Nos. 3,629,679 and 4,237,501.
Another approach is to supply stored energy for the emergency unload procedure by having a spring compressed and latched by the head positioning actuator. The spring latch is then released when a power failure occurs, providing the energy necessary to unload the heads. Such a system is exemplified by the apparatus of U.S. Pat. No. 4,005,485.
While both of these approaches work, they present a significant problem when attempting to reduce the size of a disc drive system. It is evident that a significant amount of mechanical energy is required to retract the head support structure which requires, in the case of a capacitor, a rather large capacitor. When using a spring, a fairly elaborate mechanical mechanism is required and, again, this takes up an undesirable amount of space.